\setlength{\LTpost}{0mm}
\begin{longtable}{lcccc}
\toprule
 & \multicolumn{2}{c}{Not Registered} & \multicolumn{2}{c}{Previously Registered} \\ 
\cmidrule(lr){2-3} \cmidrule(lr){4-5}
  & Voter Registration & Turnout & (Re)Registration & Turnout  \\ 
\midrule
(Intercept) & 0.018* & 0.003* & 0.041* & 0.161* \\ 
 & (0.002) & (0.001) & (0.004) & (0.007) \\ 
Information-Only Mail (T1) & 0.003 & 0.002 & 0.005 & 0.005 \\ 
 & (0.003) & (0.001) & (0.006) & (0.010) \\ 
Research-Informed Mail (T2) & 0.007* & 0.004* & 0.013* & 0.009 \\ 
 & (0.003) & (0.001) & (0.006) & (0.010) \\ 
Num.Obs. & 15760 & 15760 & 8008 & 8008 \\ 
\bottomrule
\end{longtable}
\begin{minipage}{\linewidth}
* p < 0.05\\
\end{minipage}

